feat(ui): Favourites home page (phase 1: storage + read-only grid)

New home page rendering 6 pinned-contact slots. Grid transposes to
orientation: landscape → 3×2, portrait → 2×3. Joystick claims UP/DOWN
and inner LEFT/RIGHT for grid nav; edge LEFT/RIGHT fall through to
page navigation. Selected tile inverts via drawSelectionRow, filled
tile shows contact name + unread badge, empty tile shows "+".

Storage:
- NodePrefs::favourite_contacts[6][6] (6-byte pub_key prefix per slot,
  all-zero = empty; collision probability with a real key is 2^-48)
- HomePageBit gains HPB_FAVOURITES = 11 (HPB_COUNT = 12), with a new
  HP_FAVOURITES mask bit so the page is toggleable in Settings
- New PAGE_ORDER_LEN constant decouples the persisted array length (11)
  from the page-type count; loops over page_order now use it, value-range
  checks still use HPB_COUNT
- homePageBit returns 0 for HPB_SETTINGS / HPB_QUICK_MSG explicitly
  rather than by bit-index range, so HPB_FAVOURITES (bit 11) is correctly
  treated as toggleable
- Default order inserts FAVOURITES after CLOCK; SHUTDOWN drops to the
  fallback "missing pages" append (still reachable, lands at end)

Schema sentinel bumped to 0xC0DE0002. DataStore::loadPrefsInt and
savePrefs read/write the new field. Older saves leave the field
zero-initialised (all slots empty), which is the natural starting state.

Phase 2 (Pin from Contact options) and Phase 3 (Pin from empty-slot
mini picker) wire interactions; this commit handles render + nav only.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

examples/companion_radio/DataStore.cpp

@@ -304,6 +304,8 @@ void DataStore::loadPrefsInt(const char *filename, NodePrefs& _prefs, double& no
     _prefs.page_order_set = NodePrefs::PAGE_ORDER_MAGIC;
   }
 
+  rd(_prefs.favourite_contacts, sizeof(_prefs.favourite_contacts));
+
   // Schema sentinel: bumped on layout changes. Mismatch means an older file
   // (or a different schema); rd() already zero-inits any fields not present,
   // so we just log it — next savePrefs writes the current sentinel.
@@ -394,6 +396,7 @@ void DataStore::savePrefs(const NodePrefs& _prefs, double node_lat, double node_
     file.write((uint8_t *)&_prefs.joystick_rotation, sizeof(_prefs.joystick_rotation));
     file.write((uint8_t *)&_prefs.eink_full_refresh_every, sizeof(_prefs.eink_full_refresh_every));
     file.write((uint8_t *)&_prefs.page_order_set, sizeof(_prefs.page_order_set));
+    file.write((uint8_t *)_prefs.favourite_contacts, sizeof(_prefs.favourite_contacts));
 
     // Tail sentinel — must be last. See NodePrefs::SCHEMA_SENTINEL.
     uint32_t sentinel = NodePrefs::SCHEMA_SENTINEL;